Eönwë is the banner-bearer and the herald of Manwë, and Chief of the Maiar along with Ilmarë. In The SilmarillionChristopher Tolkien added a statement that Eönwë was the "greatest of arms in Arda," meaning that he was the best with weapons and the most skilled in warfare, to explain why this character would have been chosen to lead the armies of the Valar.[1]

When the evil ValaMorgoth was defeated, Eönwë took the two remaining Silmarils and held them for safekeeping. The two remaining Sons of Fëanor took them and fled, yet Eönwë did not let them be slain.

After the overthrow of Morgoth, Sauron approached Eönwë, the former second-in-command of the Dark Lord having been shocked into (temporary) repentance. Sauron apparently hoped to secure some kind of "amnesty" from Eönwë, but the latter was not authorized to pardon a Maia like himself, so he ordered Sauron to return to Valinor to be judged by the Valar. In the event, Sauron hid in Middle-earth when Eönwë departed.

In earlier conceptions of the legendarium, detailed in The History of Middle-earth, Tolkien envisioned Eönwë, then called Fionwë, as the son of Manwë, but as the concept of the Children of the Valar disappeared from the Silmarillion tradition he became Manwë's herald instead. There is a story written there known as the "Tale of Qorinómi" about the love between him and Urwen (Arien), though in the commentary it is said that Tolkien never got to writing it.

^Online Etymology Dictionary - "Herald -c.1276 (in Anglo-Latin), "messenger, envoy," from Anglo-Fr. heraud, from O.Fr. heraut, hiraut, perhaps from Frank. *hariwald "commander of an army," from P.Gmc. *kharjaz "army" (from PIE root *koro- "war") + *wald- "to command, rule (see wield)." If so, this may be the sense of the use for both Eönwë, as Herald of Manwë and commander of the Host of the Valar, and possibly also Elrond as Herald of Gil-galad. Similarly, the 1911 Encyclopædia Britannica: "HERALD (0. Fr. heraut, herault; the origin is uncertain, but O.H.G. heren, to call, or hariwald, leader of an army, have been proposed".